Rainbow Electronics MAX3737 - Manual (Page 10)

MAX3737
Safety Circuitry Current Monitors
The MAX3737 features monitors (MC_MON, BC_MON,
PC_MON) for modulation current (I
MOD
), bias current
(I BIAS ), and photocurrent (I MD ). The monitors are realized
by mirroring a fraction of the currents and developing volt-
ages across external resistors connected to ground.
Voltages greater than V
REF at MC_MON, PC_MON, or
BC_MON result in a fault state. For example, connecting a
Multirate Laser Driver with Extinction
Ratio Control
10 ______________________________________________________________________________________
1
If any of the I/O pins is shorted to GND or V CC (single-point failure; see Table 2), and the bias current or the photocurrent
exceed the programmed threshold.
2 End-of-life (EOL) condition of the laser diode. The bias current and/or the photocurrent exceed the programmed threshold.
3 Laser cathode is grounded and photocurrent exceeds the programming threshold.
4
No feedback for the APC loop (broken interconnection, defective monitor photodiode), and the bias current exceeds the
programmed threshold.
Table 1. Typical Fault Conditions
PIN
CIRCUIT RESPONSE TO OVERVOLTAGE
OR SHORT TO V CC
CIRCUIT RESPONSE TO UNDERVOLTAGE
OR SHORT TO GROUND
TX_FAULT Does not affect laser power. Does not effect laser power.
TX_DISABLE Modulation and bias currents are disabled. Normal condition for circuit operation.
IN+
The optical average power increases and a fault occurs
if V
PC_MON exceeds the threshold. The APC loop
responds by decreasing the bias current.
The optical average power decreases and the APC loop
responds by increasing the bias current. A fault state
occurs if V
BC_MON exceeds the threshold voltage.
IN-
The optical average power decreases and the APC loop
responds by increasing the bias current. A fault state
occurs if V
BC_MON exceeds the threshold voltage.
The optical average power increases and a fault occurs
if V
PC_MON exceeds the threshold. The APC loop
responds by decreasing the bias current.
MD This disables bias current. A fault state occurs.
The APC circuit responds by increasing bias current
until a fault is detected, then a fault* state occurs.
SHUTDOWN
Does not affect laser power. If the shutdown circuitry is
used, laser current is disabled.
Does not affect laser power.
BIAS
In this condition, laser forward voltage is 0V and no light
is emitted. Fault state* occurs. If the shutdown circuitry is used,
laser current is disabled.
OUT+
The APC circuit responds by increasing the bias current
until a fault is detected, then a fault state* occurs. Fault state* occurs. If the shutdown circuitry is used,
laser current is disabled.
OUT- Does not affect laser power. Does not affect laser power.
PC_MON Fault state* occurs. Does not affect laser power.
BC_MON Fault state* occurs. Does not affect laser power.
MC_MON Fault state* occurs. Does not affect laser power.
APCFILT1
I BIAS increases until V BC_MON exceeds the threshold
voltage.
I BIAS increases until V BC_MON exceeds the threshold
voltage.
APCFILT2
I BIAS increases until V BC_MON exceeds the threshold
voltage.
I BIAS increases until V BC_MON exceeds the threshold
voltage.
MODSET Does not affect laser power. Fault state* occurs.
APCSET Does not affect laser power. Fault state* occurs.
Table 2. Circuit Responses to Various Single-Point Faults
*A fault state asserts the TX_FAULT pin, disables the modulation and bias currents, and asserts the SHUTDOWN pin.